The present invention relates to a cable branching unit to be provided in the middle of a cable, such as a submarine cable, for branching electric or optical signals transmitted through the cable.
Cable faults, which may be caused in a submarine cable system, for example, have two modes. One is called an open mode, wherein a cable is broken at a fault point and a current flowing through the cable between two land stations is interrupted.
The other is called a short mode, wherein a cable is short-circuited to the ground due to immersion of seawater, for example, at a fault point, and a current from a land station is made to flow out at the fault point.
FIG. 3 is a block diagram illustrating an example of conventional cable branching unit 301 provided in a submarine cable system for branching a cable 41 connected to a first land station 111 into two cables 42 and 43 connected to a second and a third land station 112 and 113 respectively. The cable branching unit 301 comprises a driver circuit 21 connected between the cable 41 and the cable 43 and a relay 21-1 driven by the driver circuit 21. Other components for branching transmission signals, such as a branch amplifier, are omitted to in the drawings.
The relay 21-1 has a contact point 21-1a connected to the cable 41, a contact point 21-1b connected to the ground and a common terminal 21-1c connected to the cable 42. When the relay 21-1 is driven by he driver circuit 21, the common terminal 21-1c is connected to the contact point 21-1b, and the common terminal 21-1c is connected to the contact point 21-1a when the driver circuit 21 is disabled.
When the submarine cable system is working normally, a power current is supplied between the first and the third land station 111 and 113 flowing through the driver circuit 21, which drives the relay 21-1 to connect the cable 42 to the ground and another power current is supplied between the second land station 112 and the ground.
When an open mode fault is caused at a point of the cable 43 between the cable branching unit 301 and the land station 113, the power current flowing through the cable 43 is cut and the driver circuit 21 is disabled. Therefore, the common terminal 21-1c of the relay 21-1 is connected to the contact point 21-1a and composes a power supply circuit between the first land station 111 and the second land station 112 flowing through the cable branching unit 301.
Thus, signal transmission between the first and the second land station 111 and 112 can be maintained even when the open mode fault is caused on the cable 43 branching from the cable branching unit 301 towards the third land station 113.
However, when a short mode fault is caused at a point of the cable 43 branching from the conventional cable branching unit 301, the current from the first land station 111 is left flowing out from the fault point which is grounded. Therefore, signal transmission towards the cable branching unit 301 must be suspended until the fault point is repaired. This means suspension of the power supply to repeaters provided on the cable 41 between the first land station 111 and the cable branching unit 301, in a cable system wherein transmission signals are multiplexed on the same cable for supplying the power current, thereby resulting in a breakdown of the whole system.